Process for producing nonageing super deep-drawing steel sheets



United States Patent 3,178,318 PRGCESS FOR PRUDUCING NONAGEING SUPERDEEP-DRAWING STEEL SHEETS Mineo Shimizu and Kameo Matsukura, Yawata,Fukuoka, Japan, assignors to Yawata Iron 8: Steel (10., Ltd., Tokyo,Japan, a corporation of Japan No Drawing. Filed Oct. 22, 1962, Ser. No.232,288 Claims priority, application Japan, Nov. 25, 1961, 36/ 42,894 1Claim. (Cl. 148-2) This invention relates to processes for producingnonageing super deep-drawing steel sheets to be cold-pressshaped.

For the above mentioned purpose, there has been mostly used an aluminumkilled steel. When aluminum killed steel is hot-rolled and thereaftercooled down as quickly as possible, then cold-rolled and then annealedfor recrystallization, aluminum nitride will thereby be precipitatedfrom solution and in consequence a peculiar elongated crystal grain willbe formed. This is generally recognized to the cause for the aluminumkilled steel to have a higher deep-drawability than of a rimmed steel.Further, as the nitrogen contained in the steel is fixed as aluminumnitride as described above, the steel is nonageing.

However, as there is no rimming action in casting the aluminum killedsteel, it is necessary to extend the refining time, in order to reducethe carbon content to be as low as in the rimmed deep drawing steelsheet. Moreover, due to the shrinkage pipe produced at the upper portionof the ingot, the yield of such aluminum killed steel is low and itscost is higher than of the rimmed steel.

Further, there is a defect that the surface properties of the killedsteel are inferior to those of the rimmed steel, because comparativelymany nonmetallic inclusions reside in the surface layer of the former.

On the other hand, a nonageing steel sheet can be obtained bystabilizing the nitrogen in the steel by adding vanadium and/or boron.However, as vanadium and boron have no strong deoxidating action, thesteel to which such elements have been added is a rimmed steel.Therefore, such steel sheet is superior to the aluminum killed steel inthe surface properties but is substantially the same as or ratherinferior to the latter in deep drawability.

Generally, in order to improve the press-formability of a steel sheet,it is desirable to reduce the carbon content in the steel as low aspossible. Usually it is made less than 0.08%. The other accompanyingelements are also kept as low as possible. However, if the carboncontent in the steel is made low in the refining process, the oxygencontent will increase with it, the cleanliness of the steel will bereduced and the workability of the steel sheet will be thereby ratherreduced. Therefore, it is not practical in general to reduce the carboncontent to less than about 0.03% in the melting and casting process.

An object of the present invention is to provide at a low cost and ahigh yield a steel sheet which is nonageing and whose surface propertiesand deep drawability are higher than of a steel sheet made of aconventional aluminum killed steel.

Such object of the present invention is attained by tapping a moltensteel composed of less than about 0.10% C, about 0.20% to 0.80% Mn, lessthan about 0.03% P, less than about 0.03% S, less than about 0.20% Cuand the rest being Fe and unavoidable impurities in a steel makingprocess, pouring into a mold the said molten steel after subjecting itto a vacuum degassing or casting it in a vacuum or an inert gasatmosphere so as to extremely reduce the oxygen content in the steel,making a cold-rolled steel sheet from the said steel by the conventionalmethod and then annealing the said steel sheet "ice for therecrystallization while decarburizing and denitriding it in anatmosphere containing wet hydrogen so that the carbon content in thesteel may be less than about 0.01% and the nitrogen content therein maybe less than about 0.001%.

In the thus obtained steel sheet, the oxygen content is so low thatimpurities of oxides in the steel are very low. The carbon content isless than 0.01% and the nitrogen content is less than 0.001%. Thus, thesteel sheet of the present invention is substantially nonageing and isso soft that its press-formability and surface properties are higherthan those of the conventional aluminum killed steel. Moreover, itsyield is also very high.

In the process of the present invention, the starting composition of thesteel is substantially the same as that of an ordinary deep drawingrimmed steel but the steel is decarburized and denitrided so positivelyin the annealing process as to be remarkably softened. Consequently, themanganese content in the steel sheet of the present invention can bemade higher than in the ordinary deep drawing steel and is allowed to beup to about 0.80%.

This is the more advantageous, because this manganese can be utilized toreduce the oxygen content in the steel and to eliminate the badinfluence of sulphur which is comparatively magnitudinous in theconcentrated segregated part in the head part of the steel ingot.

The present invention shall be explained with reference to an example.

Example 0.003% C, 0.008% Si, 0.32% Mn, 0.014% P, 0.015% S,

0.073% C11, 0.030% Ni, 0.016% Cr, 0.020% As,

0.002% Mo, 0.002% Ti,

0.005% 0 0.002% soluble Al,

0.001% insoluble Al,

0.0004% total N After the skin pass of a reduction ratio of 0.5%, themechanical properties of the product were as follows:

0.005% W, 0.000% Co, 0.010% Sn,

Thickness 1.0 mm.

Yield strength 11.5 kg./mm.

Tensile strength 28.1 kg./mm.

Total elongation 53.6% in mm. gage length. Erichsen value 12.2 mm.

Conical cup value 44.12 mm.

Hardness HRB 34.5 Rockwell B scale.

Grain size (A.S.T.M.)' No. 7.6.

In the ordinary refining process, when the C content was as little asthis, the 0 content was inevitably more than 0.1%. Even when the steelwas tapped with the composition of an ordinary rimmed low carbon steel,about 0.05% 0 was contained therein. But the 0 content was reduced to0.005% by the vacuum-degassing treatment. According to the A.S.T.M.pointing counting method, the nonmetallic inclusions in the steel sheetproduced by the present invention were shown as d=0.05 to 0.10% ascompared with an ordinary cold-rolled steel sheet, the nonmetallicinclusions of which are normally shown as d=0.2 to 0.41%, that is, beingreduced down to only a quarter of those in an ordinary steel sheet.

12 Moreover, the size of inclusion became very small and distributionwas uniformly dispersed.

Due to the clecarburization annealing, the C content was reduced from0.07% to 0.003%. N contentit was reduced to 0.0004% as the result ofdenitrification 'by wet hydrogen, which means a. very low N content,because even a rimmed steel containing less than 0.10% C produced in anordinary pure oxygen top blowing converter contains N normally still inthe order of 0.0017%. That is, the N content was also reduced to lessthan a quarter by denitrification'inthe process of annealing. Thus,nonageingsufiici'ent for practical use was obtained.

What we clairnisr A process for producing .a non-ageingdeep-drawablesteel sheet which comprisesvacuum-degassing a molten rimmedsteel'consisting essentially of about 0.04 to 0.10% C, about 0.20to'0.80% Mn, about 0.04 to 0.10% O and Fe whereby the oxygen content ofthe steel is reduced, casting, slabbing-and cold-rolling saidvacuum-degassed As regards the 3, rimmed steel, and then decarburizingannealing said coldrolled steel thereby producinga steel containing lessthan 0.01% C, less than 0.01% O, and less than 0.001% N.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESOpen'Coil Process, by J. Arnold, Iron and Steel Engineer, 1960, pages91-111.

DAVID L. RECK, Primary Examiner.

